I have a need to watch more than one Gmail account, and the
Gmail Watcher add-on for Firefox can watch up to 3, when using Google multiple sign in mode.
In addition to giving me a toaster style visual alert at the bottom of my screen, which I am not really that interested in, I can also have an optional audio alert, and I have it set with the most obnoxious wav file I can find.
A really nice option this has is how it can be set to automatically max your PC's volume when it plays that wav, then turn it back down to the way it was.
One thing some may not like is how it does require you to have your browser open and you to be logged in to Gmail at all times. Which is not a big deal for me, since I usually have it open, any way, and 2 of my browser tabs are dedicated to keeping me always logged into Gmail.
It's not all that complex, but it will definitely alert me when I have new mail, I can hear it in almost any room of my home (good when doing housework) and even wake me up if I happen to be napping. (which means I can nap during the day and still respond to incoming support tickets within 60 seconds

)
The post explains why it is currently out of consideration, which for many would be a minor issue.
If you need to have it repeat the alarm, it can be set to do that by unchecking the box for "only alert for new additional emails". And the frequency of the alarms would be controlled by the frequency it autochecks for new mail (the slider at the top).

And if you want to use the same obnoxious wav file I am using, you can grab it out of the .zip of my
PesterMe app.

Just imagine that playing at full volume every 60 seconds, till someone reads that e-mail, and you'll understand why I suggested it.
